add primitive expander for tree-il

add primitive expander for tree-il

* module/Makefile.am: Add inline.scm.

* module/language/tree-il.scm (pre-order!, post-order!): pre-order! is
  new. post-order! existed but was not public. They do destructive tree
  traversals of tree-il, and need more documentation. Also, add
  predicates to tree-il's export list.

* module/language/tree-il/inline.scm: New file, which expands primitives
  into more primitive primitives. In the future perhaps it will not be
  necessary, as the general inlining infrastructure will handle these
  cases, but for now it's useful.

* module/language/tree-il/optimize.scm: Move post-order! out to better
  pastures.